Induction heating is used in heating processes of metallic material strips or sheets. This type of heating envisages that some inductors, crossed by current, generate a magnetic field which induces currents in the metal, which is heated by Joule effect. In order to heat strips made of electrically conductive material a type of induction heating named “transverse flux”, may be used, in which the magnetic field produced by the inductors is mainly perpendicular to the surface of the strip itself. Typically, turn-shaped inductors, mutually arranged on two planes parallel to the upper and lower faces of the strip which is advanced, are envisaged. The conductors of the inductors facing the strip are crossed by a current, typically alternating and of the same phase, provided by a power supply unit.
The magnetic field thus generated entirely crosses the thickness of the strip, providing that the frequency of the alternating current which crosses the conductors is sufficiently low. Indeed, as the frequency increases, the currents induced on the strip will produce increasingly greater reaction fluxes, opposite to the main flux, as long as a separation of the fluxes produced on the two faces of the strip is obtained. The flux separation may be obtained at increasingly low frequencies, the greater is the thickness of the strip. In practice, the strip itself works as an electromagnetic screen.
The transverse flux induction heating apparatus makes it possible to obtain good efficiency in terms of power delivered by the power supply unit in relation to the power transferred to the strip. With respect to longitudinal flux induction heating, a transverse flux induction heating apparatus is more efficient and, being open on the side opposite to the supply of the turns, improves maintainability because it allows the strip to be extracted in case of failure. However, although advantageous from certain points of view, the technology available today for transverse flux induction heating has some disadvantages.
In particular, for the strips of a given extension, in relation to the size of the corresponding inductors, the heating along the length of the strip from one side edge of the opposite one is not homogenous. Indeed, it occurs that each side edge is heated excessively, or in all cases in non-controlled manner, and that a zone adjacent to it remains colder. In particular, the magnetic field density, and thus the power density, is higher at each edge and then drastically decreases in the zone adjacent to it and increases again, in the central zone of the strip, to the desired value to obtain the heating. Such a behavior is illustrated in FIG. 6, which shows the power density pattern, expressed in W/m, as a function of the width of the strip, expressed in meters, which is obtained with the transverse flux induction heating devices of known type. The zones in which the power density is lower can be referred to as “power gaps”. This effect is due to the fact that the current runs parallel to the plane of the turns of the inductor, following the path thereof, on the strip (the sense of the induced current is opposite to that of the turn). When the turns extend beyond the width of the strip, the induced current is forced to bend on its edge. This produces a greater heating of the edge, because the induced current, as the magnetic field, will be concentrated in a space defined by the so-called “penetration thickness”, which is as a function of frequency. The “power gap” is created in the zone in which the induced current bends because it tends to be dispersed, thinning out in an area which is about 3-4 times the “penetration thickness”.
There is a direct ratio binding the maximum power peak on the edge and the power gap. According to the known art, a method for reducing the power gap is to increase the supply frequency. This however worsens the problem of excessive heating at the edges.
It is often useful for the edges to be heated more than the center, considering that the edges tend to be colder when the strip is introduced into the induction heating apparatus. However, a controlled heating of the edges of the strip cannot be obtained with the known technology.
A further disadvantage of the currently available transverse flux induction heating devices concerns their poor flexibility for heating strips of different width. Indeed, the configuration of the heating apparatus must be adapted to obtain the optimal temperature profile for a given width of the strip, requiring complicated and costly changes in order to heat strips of different width.
US 2007/0235446A1 describes an induction device built so that each induction coil is shaped to cross the passage plane of the strip with a respective end. The configuration is such that the whole of the two induction coils entirely encloses the passage zone of the strip, thus also enclosing the zones near the passage of the edges of the strip. However, such a solution does not appear satisfactory to solve the aforesaid problems. Furthermore, it requires an excessively complex turn geometry.
The need is thus felt for a transverse flux induction heating apparatus capable of minimizing the power gaps, which makes it possible to obtain a lower, more controllable heating at the edges of the strip and which can be easily adapted to the width of the strip to be heated.